Because I want the free and immediate access to other applications. Duh!

This is precisely why the application frame is useful. When you are in full screen gray mode in Photoshop (which, by the way, is my other favorite mode to work in), you cannot get outside of that, and you cannot resize it. But the application frame, you can actually pull in the corner. Just like Apple's iApps, you simply size down the frame window a little and you can do all the dragging and dropping you want to and from anywhere else. Totally unlike the gray full screen mode that blocks out all inter-app access all the time unless you have a second monitor.

You can have your non-distracting background and you can have inter-app access, controlled by whether you maximized the frame or not. Also, n-up views and tabs are much easier to deal with in the app frame than with floating windows.